Here's my test program.

Fengnan Chang <changfengnan@bytedance.com> 于2025年8月13日周三 20:02写道：
>
> After commit 0b2b066f8a85 ("io_uring/io-wq: only create a new worker
> if it can make progress"), in our produce environment, we still
> observe that part of io_worker threads keeps creating and destroying.
> After analysis, it was confirmed that this was due to a more complex
> scenario involving a large number of fsync operations, which can be
> abstracted as frequent write + fsync operations on multiple files in
> a single uring instance. Since write is a hash operation while fsync
> is not, and fsync is likely to be suspended during execution, the
> action of checking the hash value in
> io_wqe_dec_running cannot handle such scenarios.
> Similarly, if hash-based work and non-hash-based work are sent at the
> same time, similar issues are likely to occur.
> Returning to the starting point of the issue, when a new work
> arrives, io_wq_enqueue may wake up free worker A, while
> io_wq_dec_running may create worker B. Ultimately, only one of A and
> B can obtain and process the task, leaving the other in an idle
> state. In the end, the issue is caused by inconsistent logic in the
> checks performed by io_wq_enqueue and io_wq_dec_running.
> Therefore, the problem can be resolved by checking for available
> workers in io_wq_dec_running.
>
> Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
> Reviewed-by: Diangang Li <lidiangang@bytedance.com>
> ---
>  io_uring/io-wq.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
> index be91edf34f01..17dfaa0395c4 100644
> --- a/io_uring/io-wq.c
> +++ b/io_uring/io-wq.c
> @@ -357,6 +357,13 @@ static void create_worker_cb(struct callback_head *cb)
>         worker = container_of(cb, struct io_worker, create_work);
>         wq = worker->wq;
>         acct = worker->acct;
> +
> +       rcu_read_lock();
> +       do_create = !io_acct_activate_free_worker(acct);
> +       rcu_read_unlock();
> +       if (!do_create)
> +               goto no_need_create;
> +
>         raw_spin_lock(&acct->workers_lock);
>
>         if (acct->nr_workers < acct->max_workers) {
> @@ -367,6 +374,7 @@ static void create_worker_cb(struct callback_head *cb)
>         if (do_create) {
>                 create_io_worker(wq, acct);
>         } else {
> +no_need_create:
>                 atomic_dec(&acct->nr_running);
>                 io_worker_ref_put(wq);
>         }
> --
> 2.20.1
>